1. Field of the Invention
The invention relates to an automatic cable attenuation compensation system.
As a consequence of the skin effect, coax cable attenuation expressed in dB increases with the frequency of the transmitted signal in proportion to the root of this frequency. The attenuation is further dependent on, inter alia, the length and the diameter of the cable. Such an attenuation is especially disturbing when baseband video signals are transmitted, while in the case of transmission of double-sided amplitude modulated signals, the relatively less attenuated left side-band and the relatively more attenuated right side-band can be combined to obtain a substantially flat amplitude characteristic. Such an easy compensation is not available when baseband video signals are transmitted.
2. Description of the Related Art
In applications where video signals have to be transmitted over relatively long coax cables of variable length, (semi-)automatic cable attenuation compensation systems are used. DE-A-31.48242 discloses a system which determines the cable length at power-up to switch on a fixed compensation suitable for compensating the frequency-dependent attenuation within certain limits for one type of coax cable in steps of several meters. This step-wise compensation entails the drawback that any cable attenuation which is within the resolution of the attenuation compensation system is not compensated for, so that no optimal flat frequency characteristic for intermediate cable lengths is obtained. Further, such step-wise cable attenuation compensating systems may only be operative directly after power-up, because a step-wise adjustment of the compensation at a later stage would result in a disturbed picture. This entails the drawback that any temperature-dependent attenuation caused by temperature changes cannot be compensated for.
On the other hand, U.S. Pat. No. 3,431,351 discloses an automatic frequency characteristic correction system which provides a continuous compensation of the frequency-dependent attenuation. The compensation range of such a continuously operative compensator is, however, rather small, so that no adequate compensation is obtained when the attenuation effected by the cable falls outside this range.